Reservoir Heterogeneity Caused by Diagenesis in Tensleep Sandstones, Teapot Dome, Wyoming

Peigui Yin¹, Dag Nummedal², and Qingsheng Zhang^2
1 University of Wyoming, Laramie, WY
² Colorado School of Mines, Golden, CO

Potential reservoir lithofacies in the Teapot Dome Tensleep Sandstone include eolian dune, interdune, and sand sheet, all of which are composed of fine- to very fine-grained, quartz arenites, with local concentration of dolomite sand grains. Lamination and grain size variation inherited from depositional environments caused the variation of porosity and permeability in different depositional units, whereas cementation and compaction further increased the reservoir heterogeneity.

Microcrystalline dolomite is concentrated along finer-grained laminae, reducing porosity and causing permeability directionality. Anhydrite cement exists as nodules, patches, or zones, precipitated along the remaining coarser-grained laminae after dolomite precipitation. Presence of the anhydrite cement led permeable eolian sandstones heterogeneous. Mechanical compaction distorted the ductile dolomite sand grains, damaging porosity and permeability due to filling of the intergranular space. After the diagenetic modification, the eolian dune, interdune, and sand sheet lithofacies in the Teapot Dome area can be classified into six petrophysical facies: (1) uncemented, dolomite-sand-free, dune sandstone facies, (2) cemented, dolomite-sand-free, dune sandstone facies, (3) cemented, dolomite-sand-rich, dune sandstone facies, (4) uncemented, dolomite-sand-rich, interdune sandstone facies, (5) cemented, dolomite-sand-rich, interdune sandstone facies, and (6) cemented, dolomite-sand-rich sand sheet facies. Each petrophysical facies exhibits different reservoir qualities.

Precipitation of dolomite and anhydrite cements and richness of dolomite sand grains are related to depositional environments and ground water tables. Therefore, reservoir quality of the Tensleep sandstones can be predicted based on the integrated study of depositional systems and diagenetic history.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005